Fischer-Tropsch synthesis is a process that converts syngas into hydrocarbons via catalytic reactions, which includes reactions of generation of alkanes and alkenes, together with reactions of generation of oxygen-containing compounds and water-gas shift. The reactants of Fischer-Tropsch synthesis, i.e. syngas, can be derived from coal, natural gas, coal-bed methane, biomass etc. via process of gasification or reformation. The products of Fischer-Tropsch synthesis include clean liquid fuels such as gasoline, diesel oil and kerosene, basic chemical raw materials such as short-chained alkenes, and high added-value chemicals such as high-grade wax. In recent years, as petroleum is continuously consumed and the demands for energy and resources keep increasing in the world, Fischer-Tropsch synthesis has been extensively recognized as a proven method for producing liquid fuels and high added-value chemicals. Companies such as Sasol and Shell have already established industrialized production facilities for coal to oil (CTL) and natural gas to oil (GTL). Besides, companies such as Exxon Mobil, Syntroleum, BP and so on, and gas/coal rich countries are also planning commercialization of Fischer-Tropsch synthesis. In China, there are companies such as Shenhua, SYNFUELs CHINA, Yankuang Group and etc. that have built industrial demonstration facilities for indirect production of oils from coal based on Fischer-Tropsch synthesis.
Researches focusing on Fischer-Tropsch synthesis has lasted for nearly 90 years, but there are still some problems unsolved, among which a critical one is the ineffective control of product selectivity. As the products' chain-growth follows polymerization mechanism and the product selectivity usually follows Anderson-Schulz-Flory distribution, only can relatively high selectivity be achieved for methane and heavy hydrocarbons (C21+), the selectivities for the other products are low, among which gasoline fractions (C5-C11) has a selectivity of up to about 45% and diesel (C12-C20) has a selectivity of up to about 30%. The product of commercialized Fischer-Tropsch synthesis is usually a mixture of a few oil-components. Therefore, more attention has been attracted to the development of new catalysts that can effectively control the product selectivity.
CN 101811050A disclosed a cobalt-based catalyst modified by lipophilic organic group for Fischer-Tropsch synthesis. The catalyst contains 5.0-30.0% of cobalt, 50-80% of support, 5-20% of silica and 5-40% of lipophilic group. The catalyst can be simply synthesized and enables Fischer-Tropsch synthesis with higher conversion for CO (60%), selectivity of 50% for intermediate fraction C5-C11 and less than 2% for C19+.
CN 1418933A disclosed a catalyst for selective syntheses of gasoline and diesel fractions from syngas. The catalyst comprises active metal Fe, and promoters such as Cu, K, Co, Ru or the like, and activated carbon is used as a support. This catalyst enables higher selectivity for C1-C20 hydrocarbons, among which the selectivity for C5-C20 hydrocarbons can reach 76.6%.
CN 101269328A disclosed a cobalt-based catalyst for syntheses of gasoline and diesel fractions from syngas. The catalyst mainly consists of Co and TiO2 and enables conversion of CO to be 60% and selectivity for C5-C11 to be 40% in fixed bed reactors.
CN 101224430A disclosed a cobalt-based catalyst modified by lipophilic organic groups for Fischer-Tropsch synthesis, which mainly consists of cobalt, noble metal promoters and porous silica gel. This catalyst enables conversion of CO up to 70% and selectivity for C5-C11 to be about 50%.
Martinez et al. (A. Martinez et al. J. Catal. 2007, 249, 162) discovered that higher selectivity for gasoline fractions can be achieved by using a composite catalyst which is prepared by physically mixing the conventional cobalt-based catalyst for Fischer-Tropsch synthesis and zeolite molecular sieve (H-ZSM-5 etc.). For example, composite catalyst Co/SiO2+ZSM-5 enables up to 62% of selectivity for C5-C12 when conversion of syngas is 60%. It is reported (Y. Li et al. Energy Fuel 2008, 22, 1897) that better catalytic performances can be achieved by a catalyst prepared by mixing SiO2 and ZSM-5 to form composite oxide and then loading Co thereon, by which the conversion of CO is more than 80%, the selectivity for C5-C12 reaches 55% and the content of isoparaffins is more than 10%. Tsubaki et al. (N. Tsubaki et al. Angew. Chem. Int. Ed. 2008, 47, 353; J. Catal. 2009, 265, 26) prepared a novel core-shell structured catalyst for Fischer-Tropsch synthesis by wrapping conventional Co/Al2O3 catalyst with In-Situ-Formed H-beta molecular sieve membrane. The catalyst enables Fischer-Tropsch synthesis with selectivity of 55% for C5-C12, which mainly contains isoparaffins. However, one disadvantage of the catalyst is the selectivity for methane usually exceeds 15%.
What's more, the applicant of this application lately disclosed a catalyst that enables high selectivity for the synthesis of gasoline fractions from syngas in CN 101890361A. The catalyst consists of Ru and molecular sieve and enables significantly high selectivity for C5-C11 hydrocarbon fractions. On the basis of this invention, the applicant made further development of a new catalyst for Fischer-Tropsch synthesis to prepare high-quality gasoline fractions by one-step from syngas with high selectivity as claimed in this invention.